Ground proximity warning systems as presently used in commercial aircraft have five or six modes of operation. A mode of operation refers to the criteria that is used to generate a pilot warning. For example, in a terrain closure mode of operation the altitude of the aircraft above the ground is compared with the rate of closure of the aircraft to the ground and if the closure rate exceeds a predetermined rate for a particular altitude above the ground, a warning is generated. This particular mode of operation is illustrated in U.S. Pat. Nos. 3,715,718, 3,936,796, 3,934,222 and 3,958,218.
Other warning modes include: negative climb after take-off, terrain clearance, excessive sink rate and below glide slope warning modes. A ground proximity warning system employing these types of warning modes is described in U.S. Pat. No. 3,946,358. In the type of ground proximity warning system described in this patent, various warning modes are used depending upon the phase of aircraft flight. During take-off, for instance, the negative climb after take-off mode is activated which will generate a warning if the aircraft should descend with respect to barometric altitude before reaching 700 feet above ground level. Various forms of this warning mode are illustrated in U.S. Pat. No. 3,947,810 and Bateman U.S. patent application Ser. No. 109,580, filed Jan. 4, 1980 as well as in U.S. Pat. No. 3,946,358. After the aircraft has reached an altitude of 700 feet above ground level the ground proximity warning system will switch from the negative climb after take-off mode to a terrain clearance mode which provides a pilot warning in the event the aircraft should descend below a predetermined altitude with respect to the ground. Along with U.S. Pat. No. 3,946,358, U.S. Pat. Nos. 3,944,968 and 4,030,065 describe terrain clearance type warning modes. This mode normally remains in operation until the aircraft is placed in a landing configuration with the landing gear or flaps lowered.
The applicant believes that the warning coverage of the ground proximity warning system can be improved by changing the criteria by which the switch from the negative climb after take-off mode to the terrain clearance mode is made. Specifically, this change relates to the set of circumstances where the aircraft takes off over terrain that is sloping upwardly and the aircraft never reaches 700 feet above the ground to activate the terrain clearance mode as required in the prior art systems. As a result, the aircraft could approach the ground while still climbing with respect to barometric altitude without a warning being generated.
In addition, it has been discovered that it is possible under certain conditions for a prior art system to get a nuisance or an unwanted warning in the event, for example, the aircraft climbs above 700 feet above ground thereby switching the ground proximity warning system into the terrain clearance mode but the aircraft speed is such that the terrain clearance mode will generate a warning at 1000 feet or below. Since the aircraft is at about 700 feet when the switch in modes occurs, a warning will be generated which may not be warranted by the circumstances.